GB2149455A

GB2149455A - Manual backup for electronic fuel control

Info

Publication number: GB2149455A
Application number: GB08427126A
Authority: GB
Inventors: Albert H White; Robert E Peck
Original assignee: Chandler Evans Inc
Current assignee: Chandler Evans Inc
Priority date: 1983-10-28
Filing date: 1984-10-26
Publication date: 1985-06-12
Also published as: GB8427126D0; US4793133A; CA1234697A; GB2149455B

Description

1 GB 2 149 455 A 1

SPECIFICATION

Manual backup for electronic fuel control This invention relatestofuel controls for gas turbine 70 engines, and more particularlyto electromechanical fuel controls.

Prior artfuel controlsfor gasturbine engines have incorporated devices to enable an operatorto manual ly position the main fuel metering valve and/or other 75 control elements in the event of a failure in the automatic positioning system. Examples of manual control devicesfor electronicfuel controls are illus trated in U.S. Patent Nos. 3,820,323; 4,077,203; and 4,302,931. In general, existing manual control devices 80 have not been adapted to provide a nonlinear and adjustableWf/P (fuel flowto compressor discharge pressure) schedule nor permitted manual control of the engine with the primary pilot power lever (PLA) operating in its normal quadrant. In addition, state of the art control devices have not provided a servo system with high force levels for positioning control elements wherebythe PLA lever cannot be back driven either before or during actuation of the manual control device.

In accordance with one aspect of the invention, there is provided a manual backup system foran electromechanical fuel control which may be brought into operation by an electrical powerfailure, activa tion of a switch or other occurrence such as a computer malfunction. The system of the invention, which may also serve as a positioning system, preferably incorporates opposing hydraulic pistons which are adapted to engage and confine an arm during changeoverfrom automatic control to manual control in such a manner as to effect a smooth control transition. Rotation of the power lever after engage ment results in a powered translation of one of the pistons which is in accordance with a predetermined nonlinearschedule. The arm maybe connected to a fuel metering valve or other control element via a multiplier linkage (Wf/p x P) associated with a compressor discharge pressure transducer.

A manual control system of the invention is advantageous in that it permits attainment of a nonlinear and manually adjustable Wf/P schedule and additionally allows for operation of the primary pilot power lever (PLA) in its usual quadrant. Because a system of the invention incorporates a servo system capable of providing high force levels to position control elements, the PLA lever cannot be back driven.

Objects and advantages of the invention will become readily apparentfrom thefollowing detailed description when taken in conjunction with the accompanying drawings in which an embodiment of the invention is described byway of example only. In the drawings:

FIGURE 1 is a schematic diagram of a computerto manual changeover system according to the inven tion.

FIGURE 2 is a schematic representation of the relationship between the request arm, the multiplier linkage,the compressor discharge pressure transduc er and the metering valve.

Referring to FIGURE 1, there is shown generally at a manual backup system of the invention suitable for incorporation in an electromechancial fuel control. Such a fuel control typically includes an electronic computer (notshown) which senses various engine parameters and a plurality cavity housing where the hydromechanical elements of the control are located such as the metering valve, head regulator (not shown), and pressurizing valve (not shown). In FIGURE 1,the housing 12 of the depicted manual backup system would preferably constitute a portion of the control's plural cavity housing.

A speed setting or power lever (PLA) 14 has a rotatable shaft 16 attached thereto mounted in the housing 12 and carrying a gear 18 at its opposite end. Power lever 14 is also connected to a position transducer 20 (such as potentiometer) through a mechanism comprised of a link 22, arm 24 and rotatable shaft 26 mounted in the housing 12. The position transducer 29 generates a set or requested speed signal to the computer.

An arm 28 having a rounded end 30 is disposed within a cavity 31 in the housing 12 and fixedly mounted upon a shaft 32 for rotation therewith. The rounded end 30 may be constituted by a ball bearing.

Shaft 32 is mounted for rotation in the housing 12 and is connected to other elements of the manual back-up system as shown in FIGURE 2. Suffice itto say that, for purposes of describing FIGURE 1, the angular position of the arm 28 indicates a requested Wf1p.

A hollow power piston, generally indicated at 34, is mounted in the housing 12 for axial sliding movement therein. The base 36 of the power piston is of an enlarged diameter and has its outer periphery in sliding contact with the walls of a cylindrical cavity 38.

The outer surface of the intermediate portion of the power piston 34 is in sliding engagementwith a cylindrical wall portion 40 of the housing 12 interposed between cavities 31 and 38. The power piston extends into the cavity 31 and has a face 42 which servesto isolate its hollow interiorfrom the pressure therein and is adapted to contact the rounded end 30 of the arm 28 forsetting a requested Wf/P. The power piston 34 has a bleed hole 44 extending through the wall thereof at a location spaced from the face 42.

Bleed hole 44 functions to establish fluid communication between the interior hollow portion of the piston 34 and the cavity 31. A power piston stop 46 engages the base 36 of the power piston 34 during normal control operation and is held thereagainst by the pressu re forces acting upon the power piston. An antirotation pin 47 extending from the housing into a bore in the piston base prevents piston rotation.

Mounted upon the power piston 34 in coaxial relationship therewith for rotation thereover is a rotary servo valve generally indicated at48. Rotary servo valve 48 is a short cylindrical tube 50 which carries a gear 52 in meshing engagement with the gear 18 and has a portion of its wall cut away to define a Wf1p scheduling surface 54. During changeover and manual operation the power piston 34 is adapted to moveto a position wherethe scheduling surface uncovers a portion of the bleed hole 44such thatthe piston pressureforces are in equilibrium and power piston motion will terminate in the manner of a position follower servo system. Axial movement of 2 GB 2 149 455 A 2 theservo valve48 is prevented bya datum adjustment screw56 in threaded engagementwith housing 12 which has a groove 58 at its end for receiving the periphery of the gear 52. Itshould be apparentthatthe position of the datum adjustment screw 56with respectto housing 12 will dictatethe particularWf/p for a given throttle position during manual operation.

A loading piston, generally shown at 60, is adapted to advancetoward the power piston 34 during changeoverfrom autornaticto manual operation so as to gently confinethe rounded end 30 of the arm 28 between the respective opposing faces of the con fronting pistons. The loading piston 60 is mounted in a cavity 62 in the housing 12 for axial sliding movement therein. The base 64 of the loading piston 60 is of an enlarged diameter and has its outer periphery in sliding engagementwith the wal I of the cavity 62. The intermediate portion of the piston has its outer surface in sliding engagement with a cylindrical wall 66 of the housing 12 which is disposed between the cavities 31 and 62. The face of the piston has a projection 68 thereupon which is of such a length thatthe respective faces of the pistons 34 and 60 barely touch the rounded end 30 when the projection 68 contacts the face of the piston 34. It will be appreciated that such an arrangementwill prevent damageto the rounded end during engagement of the pistons. An adjustable stop 70 screwed into the housing 12 engages the base of the piston 60 for defining a leftward limit of travel which is atthe rest postion of loading piston 60 during normal fuel control operation.

A ball valve 72 functions to supplythe power piston 34 and the loading piston 60 with the pressure necessary for operation in the manual mode. The ball valve 72 is shown in its normal position in seated engagement with a seat 74 which defines an inlet port communicating with a high pressure source Ps. The ball valve 72 is maintained in such seated engagement bythe plunger 76 of an energized solenoid 78which is screwed into the housing 12. Ball valve 72, upon cleenergization of the solenoid 78, will be forced bythe pressure Ps to seat against another inlet port 80 which communicates with a low pressure source as the plunger76 is displaced to the left by its spring preload.

Hence, the chamber82 in which the ball valve 72 is located is exposed to either low pressure or high pressure in accordance with the state of energization of the solenoid 78.

The chamber 82 is placed in fluid communication with the cavities at respective locations behind the bases of the pistons 34 and 60 by means of conduits 84 and 86, respectively. Conduit 86 incorporates a restriction or orifice 88 and conduit 84 embodies a restriction or orifice 90. The annularvolumes defined by activities 38 and 62 surrounding the respective pistons 34 and 60 are referenced to the pressure Ps of the high pressure source via conduits 92 and 94. The cavity 31 is exposed to the pressure Po of the low pressure source. Itwill be appreciated thatthe high pressure source may be fuel discharged bythe pump 125 upstream of the metering valve and thatthe low pressure source may be boost pressure.

FIGURE 2 shows the relationship between the arm and shaft assembly of FIGURE 1 to a main fuel metering valve. As depicted in FIGURE 2, the shaft 32130 is connected to a mechanical multiplier linkage 98 which is also connected to a compressor discharge pressure transducer 96 having a shaft 100 which translates in responseto changes in compressor discharge pressure. Such multiplier linkages and pressure transducers are old andwell-known devices and a description of theirdetailed construction is not essential to an understanding of the present invention norto its implementation and is omitted herefrom.

The multiplier linkage 98 has an output shaft 102, the movement of which represents Wf/p X P orWf wherein Wf is requested fuel flow and P is compressor discharge pressure. The output shaft carriesteeth 104 which mesh with the teeth of a gear 106 carried by a shaftwhich turns a rotating platetype metering valve 108. Under normal conditionsthe metering valve is positioned by a computer controlled stepper motor 110.

In the normal or automatic mode of control, with the computer driving the metering valve 108, the elements of the manual back- up system will generally be positioned as depicted in FIGURE 1. In this condition, the solenoid 78 is energized wherebythe pressure Py behind and in the interior of the power piston 34 is equal to Po, the pressure of the low pressure source. Similarly, the pressure behind the loading piston 60 is Po. The pressure Ps, the pressure of the high pressure source, acts on bases 36 and 64 of the power piston 34 and the loading piston 60, respectively to maintain them in engagementwith their stops 46 and 70 in their at rest or normal positions. Rotation of the PLA lever 14 results in the position transducer generating a signal to the computer indicative of a demanded speed, thrust, horsepower, or other quantity. During this lever movement, the servo valve 48 rotates over the power piston 34 but, of course, produces no axial movementthereof because no motive pressure differentials are engendered by such rotation. Byvirtue of the mechanical connection between the shaft32, the metering valve 108, and transducer 96,the arm 28 freely moves in accordancewith the Wf/p set bythe control when operating in the automatic mode. Itwill be aappreciated that by proper adjustmentof the stops 46 and 70, permissible maximum and minimum mechanical Wftp limits may, if desired, by imposed upon the fuel control during automatic operation. Assuming, for example,that delivery of powerto the computer is interrupted,the normally energized solenoid 78 becomes deenergized, thereby causing the plunger 76 to withdrawfrom the ball valve 72 underthe influence of its spring preload. Withdrawal of the plunger 76 results in the ball valve 72 becoming seated againstthe inlet port 80 whereby the conduits 84 and 86 are exposed to the high pressure Ps.

With the chamber 82 subjected to the pressure Ps, the power piston 34 and the loading piston 60 travel inwardly at respective velocities determined bythe sizing of the restrictions 90 and 88. The pressure Py in the interior and in back of power piston 34 assumes an intermediate value between Ps and Po asthe power piston continues itstravel. Similarly, the pressure behind the loading piston assumes an intermediate value between Ps and Po as the loading piston 60 undergoes its travel. When the bleed hole 44 reaches the scheduling surface, the intermediate pressure in 3 GB 2 149 455 A 3 the interiorand in backof the powerpiston beginsto drop; and asthe bleed hole 44 is further uncovered, the pressure Pycontinuesto drop until all pressure forces acting onthe power piston 34are in equilibrium whereby motion of the powerpiston 34terminates. 70 Subsequently, motion of the loading piston 60termin ateswhenthe projection 68 contacts the wall 42 ofthe powerpiston 34, thereby trapping the rounded end 30 between thefacesof the pistons34and 60.

During changeoverand manual operation,the pressure Py may be regarded as a variable control pressure produced between two restrictions, viz.:

restrictions 90 and 44, the latter of which is of a variable area. This control pressure functions to cause a translation of the piston to a position in which equilibrium is attained in the manner of a position follower servo system. Itwill be understood thatthe exact position which the power piston 34 assumes when the pressure forces are in equilibrium is solely a function of the position of the PLA lever 14 atthetime ofchangeover.

Restrictions 88 and 90 are sized to provide a smooth and controlled transition to the manual Wf1p. In this regard, it should be noted thatthetravel velocity of the loading piston (as dictated by restriction 88) is such 90 thatthe power piston 34 always attains its scheduled position beforethe loading piston 60 engages the rounded end 30. Such delayed engagement forestalls the occurrence of any undesirable transients during changeover. Simply stated, it should never be possi- 95 bleto havethe arm 28 driven counterclockwise bythe loading piston and then driven clockwise bythe power piston during changeover.

After engagement of the projection 68 with thewall 42 of the power piston 34, which traps the rounded 100 end 30 between the pistons, changeover is completed.

Movement of the PLA lever and consequential rota tion of the servo valve 48 produces displacement of the power piston which results in rotation of the arm.

The constant urging force of the loading piston 60 105 againstthe power piston 34 is small in comparison to the forces which displace the power piston whereby it has only a moderate impact on itsvelocity and does not significantly alterthe equilibrium position of the power pistons. 110 The rotation of the shaft 32 produces a movement of the metering valve 108 throug h the multiplier linkage 96. The stepper motor is, of course, unpowered during manual control and is backdriven by the manual system. it wil I be noted that, for similar PLA lever 115 demands, the above described system permits the PLA lever 14to be moved in the same quadrant in the manual mode as in the automatic mode thereby contributing to pilot conveniences. It is also notewor thythatthetotal range of authoritywhile in the 120 manual control mode, can be adjusted to provide engine protection over any part of the engine operat ing envelope. In addition, the PLA lever 14 could be designed for an overtravel range after engaging a detentwhereby maximum engine performance could 125 be realized.

Obviously, many modifications and variations are possible in light of the above teachings without departing from the scope of the invention as defined in the appended claims.

Claims

1. A manual backup system for an engine fuel control for a gas turbine engine having a housing, an electronically controlled metering valve in the housing for controlling fuel flow to the engine; a lever member for electronically setting a requested speed; said manual backup system comprising:

a power piston mounted in the housing for axial sliding movement therein; a loading piston mounted in the housing, in normally spaced confronting relationship to the power piston, for axial sliding movement therein; a changeover valve, mounted in the housing to effect changeoverfrom automatic to manual fuel control operation, which operates to direct a selected pressure to the pistons for causing the pistons to travel toward one another into operative engagement; an arm mounted in the housing adjacentthe pistons and operatively connected to the metering valve, the arm being confined between the pistons when the pistons are in operative engagement such that movement of the power piston produces a corresponding movement of the arm; and a servo valve mounted in the housing and operatively connected to the lever member for moving the power piston to various positions in accordance with movement of the lever membersuch thatthe axial position of the power piston is a function of the lever position.

2. A manual backup system as claimed in claim 1, further including:

First and second conduits respectively connected to the changeover valve and positioned in the housing to respectively direct pressure to the power piston and the loading piston; first and second restrictions in thefirst and second conduits, respectively, forcontrolling the velocities of the pistons during and after changeoverfrom automatic operation to manual operation.

3. A manual backup system as claimed in claim 1 or claim 2, wherein the power piston has a hollow interior portion subjected to the pressure directed thereto and wherein the piston has a bleed hole extending through the wall thereof for establishing restricted fluid communication between the interior of the piston and the exterior of the piston which is as a lower pressure and wherein the servo valve comprises:

a rotary valve mounted for rotation upon the power piston, the rotary valve having a scheduling surface thereupon adapted to partially cover the bleed hole when the piston is in an equilibrium position during manual operation; and a gearcarried bythe rotaryvalvefor rotating the rotaryvalve in responseto movements of the lever member.

4. A manual backup system as claimed in any preceding claim, further including:

a rotatable shaft mounted for rotation in the housing, the arm being fixedly connected to the shaft for rotation therewith; a mechanical multiplier having an output shaft connected to the rotatable shaft; gear means to connectthe output shaftto the 4 metering valve; a transducer for sensing compressor discharge pressure; and meansto connectthe mechanical multipliertothe 5 transducer.

5. A method of manually positioning the metering valve of an electromechanical fuel control having an electronic computer comprising the steps of:

trapping a rotatable arm between two pistons; moving one of the pistons with an hydraulically controlled servomechanism to produce rotation of the arm; actuating the servomechanism with a lever; sensing compressor discharge pressure; and positioning the metering valve with a mechanical multiplier responsive to the sensed compressor discharge pressure and the position of the arm.

6. A positioning system compriising:

a housing; a piston having a hollow interiormounted in the 85 housing foraxial sliding movement therein; a low pressuresource; a high pressure source; conduit means for establishing restrictedfluid communication between the interior of the pistonand 90 the high pressure source; cavity means to expose an exterior portion of the piston to the pressure of the low pressure source; bleed hole means to establish fluid communication between the interior of the piston and the exterior portion of the piston exposed to the low pressure source; a servo valve mounted for rotation upon the piston and having a scheduling surface adapted to control the fluid communication established by the bleed hole means such thatthe rotation of the servo valve causes a pressure imbalance on the piston which causes itto travel to a position determined bythe angularposition ofthe servo valve with respecttothe piston and the geometry of the scheduling surface.

7. A positioning system as claimed in claim 6, further comprising:

a rotatable [ever member operatively connected to the housing; and meansto drivingly interconnect the [ever member and the servo valve such that rotation of the lever member produces a corresponding rotation of the valve.

8. A positioning system as claimed in claim 7, wherein the interconnecting means comprises:

a shaft mounted for rotation in the housing, the shaft being connected to the lever member; a first gear carried bythe shaft; and a second gear carried by the servo valve in meshing engagement with the first gear.

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9. A manual backup system for an engine fuel control for a gas turbine engine, which control is of the type having: a housing, a metering valve in the housing forcontrolling fuel flowto the engine, a motor operatively connected to the metering valvefor positioning the metering valve, an electronic computerforsensing various engine parameters and controlling the motor and a lever memberfor generating a signal to the computer indicative of a demanded GB 2 149 455 A 4 quantity, said manual backup system comprising:

means responsiveto movements of the lever member for setting a requested fuel flowto compressor discharge pressure ratio as a function of the position of the lever member; transducer means forsensing compressor discharge pressure; multiplier meansfor multiplying the setfuel flowto compressor discharge pressure ratio and the sensed compressor discharge pressureto obtain an output representative of requested fuel flow; and means responsiveto the output of the multiplier meansfor positioning the metering valve.

10. A manual backup system as claimed in claim 9, further including:

valve means to render operative the lever member movements responsive means.

11. A manual backup system as claimed in claim 9 or claim 10, wherein the motor is a stepper motor and wherein the metering valve is of the rotating plate type.

12. A manual backup system substantially as herein described with reference to the accompanying drawings.

13. A method of manually positioning the metering valve of an electromechanical fuel control, substantially as herein described with referenceto the accompanying drawings.

14. A positioning system substantially as herein described with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majestys Stationery Office, 8818935, 6185, 1. Published at the Patent Office, 25 Southampton Buildings. London WC2A lAY, from which copies may be obtained.